Many wireless transmitter systems require that a specified radio frequency (RF) output power be monitored. For example, some portable wireless transmitter systems require that power delivered to the radiating antenna be monitored. Further, many such systems are required to adjust the transmitted power to achieve a specific level depending upon signal strength. To meet the aforesaid requirements, the system architectures generally incorporate a closed-loop power control scheme. Typically, this scheme requires a “sampling” of the RF power amplifier output power that is subsequently fed back to predetermined control circuitry which generates a control signal that adjusts the output power until it is within the specified power level. Such sampling of the output power is disadvantageous in that it increases the insertion loss between the output of the power amplifier and the radiating antenna. Therefore, sampling of the output power decreases the available output power from the power amplifier and reduces the overall talk time available to portable wireless devices. Talk time is a measure of the time a portable transceiver can be in the “talk” mode before the battery is fully depleted. The power amplifier consumes the majority of the current and therefore dominates in the calculation of talk time.
A common technique for sampling the output power includes the use of a directional coupler on the output of the power amplifier. The power coupled from the main signal path is diode detected to generate a video signal proportional to the amplitude of the RF voltage delivered to the antenna. Use of directional couplers, however, adds loss to the system, forcing the power amplifier to consume more power thereby reducing the talk time of the associated radio unit. In typical applications, the aforesaid loss is often 5-10% of the power amplifier output power and relates to a direct loss in available talk time.
Another common technique for detecting the output power includes measurement of the current consumed by the power amplifier. This current is directly related to the output power generated by the power amplifier and is also fed back to predetermined power control leveling circuitry. This technique is also disadvantageous due to the loss associated with the current measurement. This current measurement generally requires that a series “dropping” element be added between the associated battery and the power amplifier bias input. The voltage across this element will determine the current entering the power amplifier (for a known resistance across the element). In typical applications, the voltage across the dropping element will be about 3% of the total battery voltage. Because this is a loss in the input power to the power amplifier, the loss of talk time will be even higher than 3% due to the less than 100% dc-rf conversion efficiency of the power amplifier. For example, if the power amplifier efficiency is 60%, then the talk time loss will be 3/0.6+L or 5%.
Thus, there remains a need for a new and improved technique for current sensing associated with RF amplifier power detection.